Focused ion beam systems are used in a variety of applications in integrated circuit manufacturing and nanotechnology to create and alter microscopic and nanoscopic structures. Focused ion beams can use a variety of sources to produce ions. Liquid metal ion sources can provide high resolution, that is, a small spot size, but typically produce a low current and are limited in the types of ions available. The different ion species have different properties, which make some ion species more preferable than others for specific applications. For example, whereas helium ions are useful for imaging or light polishing, xenon ions provide higher milling rates that are useful for bulk processing. Plasma ion sources can produce ions of many different species and at larger currents, but often cannot be focused to as small a spot.
Plasma ion sources ionize gas in a plasma chamber and extract ions to form a beam that is focused on a work piece. Many different types of gases can be used in a plasma ion source to provide different species of ions. As ions are extracted from the plasma source to form the beam, the gas in the plasma must be replenished to maintain the plasma. Typically a gas inlet for a plasma ion source has a small opening through which gas is supplied to maintain the pressure in the plasma chamber. Because the gas is used very slowly, the small opening to replenish the gas is very small. When a user desires to change the gas in the plasma chamber to form a beam from a different ion species, it can take up to 30 minutes to remove one gas and fill the chamber with a second gas. This is an unacceptably long time for many applications that process a work piece sequentially using different process gases.
FIG. 1 shows a typical prior art inductively coupled plasma source 100 for use with a focused ion beam system such as the one described in U.S. patent application Ser. No. 12/373,676 for a “Multi-source Plasma Focused Ion Beam System,” which is assigned to the assignees of the present application and is hereby incorporated by reference. Gas is provided to a plasma chamber 102 within a source tube 103 from an external gas feed line 104 through a gas filter 106 and then to a capillary tube 108 with a flow restriction 110. Energy is fed into the plasma chamber 102 from RF power supply 113 by antenna coils 114 and ions are extracted through a source electrode aperture 116 in a source electrode 118 by extractor electrode 120. The gas conductance into and out of the plasma chamber 102 is through the flow restriction 110 in the capillary tube (at the top of the source tube 103) and the aperture 116 (typically 175 μm in diameter) in the source electrode 118. Pump 122 connected to gas supply line 104 through valve 123 removes gas from plasma chamber 102 through capillary 108 and gas supply line 104. An ion column pump (not shown) extracts gas from plasma chamber 102 through source electrode aperture 116. Multiple gas sources such as gas storage 130A, gas storage 130B, gas storage 130C and gas storage 130D supply gas into gas supply line 104. A beam voltage supply 132 supplies a high voltage to the plasma in chamber 102 and an extraction voltage supply 134 supplies a voltage to extraction electrode 120. Extracted ions or electrons are focused by focusing electrode 136. Additional details of the focusing column and sample chamber are not shown.
To remove a gas from the interior of the plasma chamber, the gas feed line 104 itself may be pumped as shown to remove gas in the source tube above the flow restriction 110 in the capillary tube 108. The volume of the FIB system below the source electrode 118 may also be adequately pumped using the main chamber vacuum pump(s) (not shown).
Because both the source electrode aperture 116 and the flow restrictor 110 have small diameters and correspondingly very low gas conductances, it is impossible to rapidly pump out the interior of the source tube 103. This is a disadvantage, especially for a production FIB system. First, it may take a much longer time to pump out a first process gas from the source tube 103 before the base pressure is low enough to introduce a second process gas. Insufficient purging of the gas can lead to contamination of the plasma through ionization
Second, it may take a long time, during bakeout, to pump away contaminants which are thermally desorbed from the interior walls of the source tube 103.
What is needed is an ion source for a focused ion beam system that provides for rapid changes of gas.